Novel ZrO2-glycine nanocomposite as eco-friendly high temperature corrosion inhibitor for mild steel in hydrochloric acid solution

We report the green synthesis of novel ZrO2-Glycine nanocomposite referred to as ZrO2-Gly NC followed by its characterization using X-ray diffraction (XRD), Fourier transforms infrared (FT-IR) spectroscopy, SEM/EDX, and transmission electron microscopy (TEM) techniques. Further, the inhibition effect of the varying concentration of ZrO2-Gly NC on the corrosion of mild steel (MS) in 1 M HCl was investigated by weight loss and electrochemical measurements at 40–80 °C. The percentage inhibition efficacy of NC increased with the increase of concentration and temperature and reached about 81.01% at 500 ppm at 70 °C which decreased at 80 °C and exhibited 73.5% inhibition efficiencies. According to the polarization measurements, the investigated ZrO2-Gly NC works as a mixed-type inhibitor with predominantly inhibiting cathodic reaction. Also, the adsorption isotherm analysis indicated that the adsorption was spontaneous and followed the Langmuir adsorption isotherm. Furthermore, the contact angle measurement revealed the water-repelling property of the investigated inhibitor. The surface morphological study via SEM–EDS micrograph affirmed the appearance of a smooth surface in presence of inhibited media suggesting the formation of protective film by the adsorption of ZrO2-Gly NC on the surface of the MS even at higher temperature.


Scientific Reports
| (2022) 12:9274 | https://doi.org/10.1038/s41598-022-13359-y www.nature.com/scientificreports/ stirrer. The extract was purified employing Whatman's filter paper, and the supernatant was extracted and kept in a refrigerator. Briefly, 20 ml of fresh extracted Eucalyptus bark extract (5 mg/ml) was cultivated in DI water and poured into 70 ml of aqueous zirconyl nitrate (ZrO(NO 3 ) 2 ) solution. The colloidal solution was obtained following 30 min of vigorous stirring at room temperature. Precipitation of the colloidal solution was accomplished by adding ammonia at a pH of 7.0, centrifuging at 8000 rpm for 10 min, and rinsing twice with DI water. The resultant solution is evaporated up to dryness in a vacuum oven and crushed into a fine powder using a mortar pestle. The powdered sample was sintered for three hours at 600 °C in a muffle furnace. The synthesized ZrO 2 NPs were kept in a dry and dark condition till further utilization.

Synthesis of ZrO 2 -Gly nanocomposite.
Ultrasonication was used to functionalize the ZrO 2 NPs with glycine.The ZrO 2 NPs (100 mg) dispersion in 50 ml distilled water was achieved employing ultrasonication for 30 min. The resulting suspension was then infused with 100 ml of a 10% glycine solution (in 1% acetic acid).
Following that, the sample was continuously stirred for 24 h at 40 °C before being subjected to ultrasonication for another 120 min (Fig. 1). It was washed three times in distilled water to get rid of the extra glycine. The ZrO 2 -Gly nanocomposite was then dried. The ZrO 2 -Gly nanocomposite was further functionalized with glycine to enhance its dispersibility in aqueous environments.
Corrosion studies. Materials and reagents. The working electrode (WE) in this experiment was mild steel and the composition is given in our earlier publications 29,30 . For weight loss measurement, the steel was cut into sheets with dimensions of 2.5 cm × 2 cm × 0.1 cm from a large strip. The WE were then polished with emery sheets in a range of grits (#320 to 1200) before being sonicated in 50% ethanol, dried, and stored in a desiccator.
All corrosion experiments were carried out in 1 M HCl (Fisher Scientific, India), which was generated from HCl stock (37%, reagent grade).
Methods. Weight loss measurement was executed to study the anticorrosive impact of NC at 5 temperatures i.e., 40,50,60,70, and 80 °C. The specimens were immersed in a 250 ml beaker containing the aggressive solution prepared without and with specific concentrations of NC for 6 h. After the soaking process, the steel specimens were removed from the analysis solution, corrosion product removed, cleaned properly using deionized water, dried, and weighed again precisely. All these tests were employed in triplicate and the standard deviation was for the three measurements has been given in the respective tables. Three-electrode cells along with potentiostat/galvanostat supplied by Autolabwere used forthe conduction of the electrochemical measurements. The platinum wire, Ag/AgCl, and MS were utilized as reference, auxiliary www.nature.com/scientificreports/ and working electrodes, respectively. The polarization measurements with a constant potential sweep rate of 0.16 mV s −1 were recorded under potentiodynamic conditions within the range of -250 to + 250 mV. Impedance plots were also obtained by the automatic shift of potential from 10 −2 to 10 5 Hz with perturbation of 10 mV. To ensure suitable reproducibility each experiment was accomplished three times. Before each experiment, the working electrode was immersed for 30 min in the test solution to achieve a quasi-stable condition. All experiments were conducted at 40, 50 60, and 70 °C (within ± 1 °C) using a water bath to regulate the temperature of the cell. The measurements for each experimental condition were repeated thrice to confirm data reliability and reproducibility, and the average values were documented. The corrosion protection efficiencies of NC by the PP and EIS method were predicted using the following formulas: where I corr (0) and I corr (i) are the corrosion current densities without and with the NC, R p (0) and R p (i) are the polarization resistances of MS without and with the NC, respectively.
Double-layer capacitance (C dl ) is determined by means of the component of constant phase element (CPE) i.e., Y 0 (proportional factor) and n (phase shift) following equation.
where ω max = 2πf max (f max refers to the maximum frequency at which the imaginary part of the impedance has a maximum value).
The FT-IR spectra for synthesized NC and the adsorbed NC on the MS surface after 6 h immersion in 1 M HCl was obtained. The film was scrapped, combined with KBr, and converted into pellets for the adsorbed Fe-NC inhibitor spectrum.
With the aid of KRUSS GmbH Germany (model FM41Mk2 Easy drop) wettability analyzer, the wetting properties of the tested coupons were investigated to determine the hydrophobicity of the optimum concentration of NC relative to corroded WE. Furthermore, for morphological tests, the MS samples were subjected to aggressive media (1 M HCl) in the absence and presence of NC for 6 h and then were washed with distilled water and dried, and then analyzed by SEM/EDX. SEM instrument (model: JEOL JSM-6510LV) attached with EDX of model INCA, oxford was used for this purpose.
FT-IR analyses were performed to determine the attachment of glycine to the surface of the ZrO 2 nanoparticle. Figure 3 illustrates the FT-IR spectra of ZrO 2 -Gly and ZrO 2 nanoparticles in the 4000 cm −1 to 400 cm −   , respectively. This reflection shows that the glycine molecule exists in ZrO 2 -Gly as a zwitterionic form. Correspondingly, the peaks revealed at 847.97 and 1323.99 cm −1 are assigned to CCN and CH 2 stretching groups. The existence of ZrO 2 in the ZrO 2 -Gly was verified through the peak at 535.99 cm −1 and 453.97 cm −1 , respectively. The slight shifts in the various peaks of prepared ZrO 2 NPs and ZrO 2 -Gly could be due to the interaction between Gly and ZrO 2 NPs.
An investigation of the morphology of the synthesized ZrO 2 -Gly NC was carried out using SEM and HRTEM. Figure 4A depicted a scanning electron microscope picture of ZrO 2 -Gly NC with a wrinkled microstructure of the size of 1 micron, which was interconnected by a significant number of nanoparticle connections. EDX analysis (Fig. 4B) indicated the existence of C, N, O, and Zr. Aside from that, Fig. 4C illustrates that elemental mapping evaluation gives the direct elemental distribution of the samples, which determines the homogeneous dispersion of Carbon (red), Nitrogen (yellow), Oxygen (green), and Zirconium atoms (blue) in ZrO 2 -Gly NC. Figure 4D shows the results of the TEM analysis used to evaluate the functionalization of glycine on the ZrO 2 nanoparticle surface. Figure 4D shows that the surface of a wrinkled nanoplate morphology of ZrO 2 nanoparticle with an average particle size of 12 nm was effectively wrapped with glycine. It has been established that ZrO 2 -Gly NC was synthesized. TEM images of ZrO 2 -Gly NC revealed the black uniform dispersion of ZrO 2 nanoparticles implanted on the surface of the glycine molecule.
Weight loss measurement. Effect of concentration and temperature. The results of weight loss measurements for MS in 1 M HCl with different concentrations of NC for 6 h at different temperatures are shown in Table 1.The experimental parameters like corrosion rate (CR), inhibition efficiency (IE%), and surface coverage (θ) are calculated by the following equations 31 : Here, K-3.45 × 10 6 is a constant, W-weight loss in gram, A-total surface area in cm 2 , T-is the exposure time in h, ρ-density (7.86 g cm− -3 ), IE(%)-percent inhibition efficiency, CR 0 and CR i -corrosion rates in absence and presence of inhibitor, θ-surface coverage.
From Table 1, it is found that with the increase of inhibitor concentration up to 500 ppm, the corrosion rate reduces appreciably, consequently, the inhibitor efficiency and the surface coverage also increase which suggests that the increase of concentration leads to an increase in the adsorption capability of the inhibitor. Above 500 ppm, the increase in ZrO 2 -Gly NC concentration does not affect the inhibition efficacies much. This phenomenon indicates that the MS surface may have been already fully covered by inhibitor molecules with the 500 ppm corrosion inhibitor in solution at 40 °C. This may be due to the saturation of adsorption of inhibitor molecules on the metal surface which exhibits a negligible increase in protection efficacies 18 .  www.nature.com/scientificreports/ The effect of temperature for MS corrosion in the temperature range of 40-80 °C in the test solution was also evaluated using the weight-loss method. From the data in Table 1, it can be concluded that by raising the temperature up to 70 °C, the corrosion rate increases quickly without and with the inhibitor. However, the increase in corrosion rates in the inhibited medium is less than in the blank solution which is suggestive of ZrO 2 -Gly NC adsorption on the metal surface and therefore, blocking the active corrosion sites responsible for the corrosion. This adsorption of NC leads to an increase of IE (%) with increasing the temperature and reaches to maximum up to81.01% at 500 ppm at 70 °C. This behavior is suggestive of chemisorption 32 . However, at 80 °C the inhibition efficacies decrease slightly and reached 73%. The phenomenon may be due to the desorption of inhibitor molecules on the metal surface.  www.nature.com/scientificreports/ Activation parameters. The temperature increase has a great influence on the corrosion rate, the adsorption equilibrium and kinetics. These thermodynamic activation functions were obtained through Arrhenius and the transition-state Eqs. (7) and (8) 33 : where A denotes the pre-exponential factor, T absolute temperature in Kelvin, and R, the Gas Constant in JK −1 mol −1 .
where N is the Avogadro's number and his the Planck's constant. The values of activation energy (E a ) calculated from the log CR vs. 1/T plot (Fig. 5a) are given in Table 2. The lower value of the activation energy (E a ) of the process in an inhibitor's presence when compared to that in its absence is attributed to its chemical adsorption; the physical adsorption is pronounced in the opposite case 34 . The lower value of E a in the presence of NC compared to that in its absence and the increase of its IE (%) with temperature increase can be interpreted as an indication of chemical adsorption 35 .
The entropy of activation (∆S*) and enthalpy of activation (∆H*) were computed using Transition State Eq. (8). From the slope (− ∆H*/2.303R) and intercept [log(R/Nh) + ∆S*/2.303R] of Fig. 5b, the values of ∆H* and ∆S* were determined respectively. All the calculated values of ∆H* and ∆S* are displayed in Table 2. The positive sign of (ΔH*) infers that the process of the mild steel dissolution is endothermic, which implies the difficulty of the mild steel corrosion in the acid solution protected with a synthesized inhibitor. The decrease in ΔS* values ( Table 2) in presence of inhibitor-protected acid solutions indicate that entropy decreases on forming the transition state, which indicates an associative mechanism 36 .
Adsorption isotherm and thermodynamic parameters. To assess the probability of an adsorption activity on the surface of the mild steel specimen, the adsorption isotherms were computed. We investigated several isothermal adsorption equations which relate the inhibitor concentration and the surface coverage. To that end we tested the isothermal adsorption models of Langmuir, Temkin, and Freundlich. In comparison to other isotherm models, the regression coefficients for Langmuir adsorption isotherm are closer to unity (Fig. 6), therefore, showing it   where C, θ, K ads are the concentration of inhibitor, surface coverage area, and equilibrium constant, respectively. Herein, K ads , obtained from the intercept of the Langmuir isotherm, displays higher values, given in Table 3, meaning that NC can preferentially adsorb on the mild steel surface and thus deliver great IE values. The corresponding adsorption isotherm curves and related thermodynamic parameters such as standard Gibbs free energy of adsorption, ΔG°a ds , standard adsorption enthalpy (ΔH°a ds ), and standard entropy of adsorption (ΔS°a ds ) are presented in Fig. 6 and Table 3 respectively, which are calculated using Eqs. (10)-12 38 :   The depressed semicircular loops of Nyquist plots were obtained at all the investigated concentrations and temperatures with the center under the real axis is typical behavior for solid-metal electrodes with frequency dispersion of the impedance data due to roughness and other in homogeneities on the sample surface 26 . Nevertheless, the spectra for various concentrations of inhibitors at all studied temperatures are identical in the shape, suggesting that there is no modification in the corrosion process 40,41 . Nonetheless, the existence of the NC in the acid solution, depending on the concentration, has an impact on the size of the semicircular curves, which implies that the introduction of NC has delayed the WE dissolution rate. Furthermore, the radius of the semicircle decreases in the absence of NC with increasing the temperature from 40 to 70 °C due to the penetration of corrosive media which facilitates the MS corrosion. In comparison to the blank, when NC was added to the corrosive solution, the impedance radius increased dramatically and continued to increase as the NC concentration increased at each temperature. As a result of this occurrence, an adsorbed layer formed on steel, which gave good corrosion resistance. Figure 10 demonstrates the equivalent circuit model which is used to fit the electrochemical impedance data. The experimental and computer fit results of the Nyquist and Bode plots for MS without and with NC are shown in Fig. 11a-c. The fitting standard of the equivalent circuit was checked by the chi-square (χ 2 ) values. The small χ 2 values i.e., 10 -4 , suggest satisfactory fitting of obtained impedance spectra to the proposed equivalent circuit. The fit results are also agreeable with the experimental data. The derived (R s , R p , n, Y 0 ) and computed (C dl and IE) parameters are displayed in Table 4. Here polarization resistance is R p , and the exponent n-is the phase shift representing the degree of deviation of the CPE exponent, when n = 1 it represents ideal capacitive behavior. When n = 0.5 it is indicative of diffusion control 42,43 and it represents pure resistive behavior when it is equal to 0. The values of n approaching 1, suggest the characteristics of the pseudo capacitor are more apparent. Hence, when using capacitance (C) on the actual EIS, it is difficult to have a good fitting effect. Constant phase element (CPE) is used instead of C and defines CPE impedance 44 .
Here, CPE is called a dummy capacitor, n represents the degree to which the dummy capacitance deviates from the pure capacitance. In the present study the CPE exponents (n) are in the range of 0.890 to 0.9159 at 40 °C, 0.894 to 0.910 at 50 °C, 0.896 to 0.920 at 60 °C and 0.90.3 to 0.937at 70 °C and these values are more than the blank solution (0.88721, 0.75436, 0.72865, 0.70781 at 40, 50, 60 and 70 °C, respectively). The increase in values n implies that the NC reinforce the homogeneity rate of MS by forming an adsorptive film over the steel surface.
The relation time constant (τ) 45 of the adsorption process is calculated following Eq. (14) The τ decreased at all studied temperatures i.e., 40, 50, 60, and 70 °C respectively in presence of NC as compared to blank, indicating that charging and discharge levels for metal-solution interfaces decreased significantly 46 . www.nature.com/scientificreports/ Moreover, the values of R p and C dl as could be seen in Table 2 vary in the opposite direction with NC concentrations, i.e., R p increase and C dl decrease with an increase in inhibitor amount. The pattern found is in line with those identified by other researchers 47 . This finding demonstrates that the NC develops a protective film that prevents the dissolution of the mild steel.
It is seen that the values of R p in the inhibitor-containing solution are much larger than that in inhibitor-free solution, especially at 500 ppm. For example, the value of R p increases from 20.4 to 29.1 Ω cm 2 after adding 50 ppm, and the corresponding IE (%) reaches 29.86%. By increasing the concentration of NC to 500 ppm, the value of R p increases to 77.8 Ω cm 2 and the corresponding IE (%) reaches 73.79%. Figure 12a-d shows corresponding Bode plots for MS at different temperatures.In the Bode diagrams, log [Z] and phase angle (ϕ°) are plotted against log f. The Bode graphs illustrate that at each concentration of the NC, there is just one phase maxima suggesting that corrosion process is occurring through one-time constant 48 . For pure capacitive behavior, a linear relationship between log f and log Z should be produced with a slope of − 1. The corrosion prevention properties of studied NC are confirmed by the increase in the absolute value of the impedance at low frequencies, which is due to the formation of an inhibitory film that prevents MS to come in the contact with the corrosive solution.The slope close to − 1 and phase angle (θ)close to 90° suggests the capacitive behavior and the modulus has increased as compared with the blank with the addition of ZrO 2 -Gly NC and thus suggest that the perturbations induced by the frequency changes on the metal electrode result in the protection of iron 49 .    Fig. 12a-d. The potentiodynamic polarization parameters such as corrosion potential (E corr ), corrosion current density (i corr ), and anodic and cathodic Tafel slope (β a and β c ) are presented in Table 5. The E corr values given in the Table 5 confirm the mixed type of behavior of NC. The corrosion potential of inhibited systems is displaced toward a more negative direction relative to the blank. The negative displacement of E corr values can be successfully explained by the fact that NC produces a more beneficial effect on the cathodic type  www.nature.com/scientificreports/ of reaction i.e., H + ion reduction than that of the anodic reaction i.e., MS dissolution 50,51 . This implies that the protective inhibitor film formed on the MS surface suppresses the transfer of H + ions to the cathodic site of mild steel surface and hereby decreases the evolution rate of H 2 gas. As seen in Fig. 12, a decline in the cathodic and anodic current density values is observed in rising NC concentrations. This phenomenon can be explained by the adsorption of NC to active centers in both cathodic and anodic regions. The similar trend of Tafel curves could be observed from different temperatures. Further, as the temperature increases, the corrosion rate increases both in the HCl solution without and with the inhibitor but the increase in corrosion rate is less as compared to blank at each concentration. Table 5 shows that i corr values increases as temperature rises for the blank solution. Analysis of the results in Table 4 indicates the pronounced increase in i corr values for a blank solution as the temperature rises. However, in the presence of NC molecules, the i corr of MS decreases at any given temperature at all studied concentrations due to the increase of the surface coverage degree 52 . The percentage of inhibitor efficiency (IE%) is calculated from Eq. (1) and tabulated in Table 5. The inhibition efficiency increased with temperature according to the following order: 70 °C > 60 °C > 50 °C > 40 °C. The higher inhibition efficiency at 70 °C may be attributed to the higher electronic density of the functional groups resulting in the formation of a more adsorbent, and as a result, it caused the highest inhibition. This represents a greater chemical adsorption contribution suggesting that the bond between the inhibitor molecules and the metal surface is more likely to be electron sharing. When the polarization potential continues to increase, it can be observed that the curves are close to coinciding. This is likely due to the rapid dissolution of MS at the strong polarization potential, causing the desorption of NC from the metal surface 53,54 . Contact angle measurement. Contact angle measurement is a normal tool to understand the wetting capacity. A contact angle less than 90° usually indicates that wetting of the surface is very favorable, and the fluid will spread over a large area of the surface. Contact angle greater than 90° generally mean that wetting of the surface is unfavorable, so the fluid will minimize contact with the surface and form a compact liquid droplet 55 . In the present study, the water contact angle (θ water ) value of uninhibited MS surface reached 44.98° suggesting   www.nature.com/scientificreports/ a greater affinity of MS surface towards the corrosive environment (Fig. 13a) while NC inhibited MS surface showed the θ water value at 72.2° (Fig. 13b). As compared to the uninhibited surface, higher water contact angle values on the NC adsorbed MS surface indicate that NC intensified the hydrophobicity means the metal surface is now less prone to water 56 . The hydrophobicity of NC molecule slows down the penetration of the water-soluble corrosive components at the MS surface 57 .
Scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS) study. Figure 14 illustrates the SEM images of test coupons. Figure 14a shows the topology of the MS plain surface without immersion. It shows a defect-free surface with some scratches arising out of polishing. Figure 14b, d, f, and h shows the SEM micrographs of the corroded surface formed in 1 M HCl solution at different temperatures (40-70 °C). The surface at 40 °C becomes rough and uneven, and some pits were also observed (Fig. 14b) due to uncontrolled attack of aggressive media on the metal surface leading to the dissolution of metal. Furthermore, as the temperature rises from 40 to 70 °C (Fig. 13d,f,h), the surface becomes increasingly rough due to increased corrosion rates and is therefore severely damaged. In contrast, the surface becomes smooth and uniform after 6 h immersion in 1 M HCl with 500 ppm NC (Fig. 14c) at 40 °C. The further increase in temperature leads to increase protection capabilities of studied NC which are exhibited by the smooth surface of mild steel  www.nature.com/scientificreports/ ( Fig. 14e,g,i) and the highest inhibition capabilities were seen at the temperature of 70 °C (Fig. 14i). The surface examination of the test coupons by SEM studies, therefore, validates the capability of NC to protect the metal surface appreciably at elevated temperatures. Figure 15a shows the EDS profile of MS plain surface without immersion which shows elemental composition in wt% as Fe-93.86, O-1.69, C-3.89, Cr-0.22, Si-0.34 pointing towards fewer oxides and more iron content in the elemental state. Figure 15b shows the EDS profile of MS with an immersion time of 6 h in HCl without NC with an elemental composition in wt. % as Fe-85.13, O-7.73, C-6.99, and Cl-0.15 indicating oxides formation with low elemental iron, and the surface is markedly corroded. NC inhibited surface (Fig. 14c) shows mild steel surface with its elemental wt% composition as Fe-92.0, O-4.70, and N-2.32 signifying less tendency of iron oxides formation and retention of the original iron.
Inhibition mechanism. Amino acids strongly bind to the ZrO 2 surface in the dissociative adsorption configuration mainly because of the covalent interaction between the oxygen atoms of the amino acids and the Zr atoms. Glycine strongly binds to Zr6f ion either via the amino (-NH 2 ) or the carboxyl group (-COOH). However, the most stable binding mode for glycine occurred via the interaction of the two oxygen atoms of the carboxylic group with two Zr6f sites. According to the adsorption isotherm results as mentioned above, the existence of both chemical and physical interaction is confirmed between ZrO 2 -Gly NC and the steel surface. Mild steel is charged positively on its surface in the presence of acidic media 58,59 and therefore, chloride ions from aggressive media get adsorbed on the metal surface (Fig. 16a). NC is present on the surface in both protonated and neutral forms. ZrO 2 -Gly NC molecules are protonated in the acid solutions through their heteroatoms. These cations are electrostatically drawn onto the MS surface already hydrated with Cl − (HCl medium) (Fig. 16b). At the same time, NC can be chemically adsorbed on the steel surface on account of coordination bonds formed by transfer of lone pair electrons of N-containing side groups to Fe atoms, as well as back-donation of electrons between Fe